Select Interior Face, how to make it work?
This tool is very limited. All it does is exactly in it's description:



Difference between an edge with 2 face users and edge with 3 (or more) face users:



All this tool does is for every face it checks if all it's edges have more than 2 face users. If yes, such face is selected.

This tool does not 'think' spatially and doesn't understand volumes at all. Here is an example when it will succeed and when it will fail: